1. Field of the Invention
The present invention relates to a semiconductor device having a thin film resistor whose resistance value is adjusted by means of laser trimming.
2. Description of the Related Arts
Hitherto, integration of a thin film resistor on a semiconductor substrate has made it possible for a semiconductor device to be made smaller and less costly. For adjustment of the resistance value of the thin film resistor, a laser trimming method is generally used which adjusts a resistance value by fusing the thin film resistor with laser beams irradiated on it. Also, a two-layered passivation film is used for the protection of the thin film resistor, as proposed, for instance, U.S. Pat. No. 4,217,570. As shown in FIG. 1, a silicon oxide film 3 is formed on the thin film resistor 4 for the trimming capability, and a silicon nitride film 2 is formed on the silicon oxide film 3 for the environmental resistance. FIG. 1 also shows a silicon substrate 6, an underlying oxide film 5 such as silicon oxide film, and an aluminum electrode 7.
An important thing for the laser trimming process is to adjust the quantity of laser energy absorbed by the thin film resistor to an optimum level. If the absorbed energy is too little, the thin film resistor will fuse insufficiently, and if it is excessive, the insulating films or semiconductor substrate will be damaged.
In FIG. 1, the laser beams transmitted through thin film resistor 4 are partly reflected by the surface of underlying oxide film 5 and by the surface of silicon substrate 6 and are absorbed again, while interfering with the transmitted light, by thin film resistor 4. There also occur reflections and interference on the surface of thin film resistor 4 and the surfaces of surface passivation film 2 and 3. The total energy of the laser beams absorbed by the thin film resistor 4 varies depending on the thickness of each film composing a semiconductor device. Therefore, an optimum adjustment of the thickness of each film is required to perform laser trimming in a stabilized manner.
However, due to such factors as nonuniformity in a production process, film thicknesses can vary for each wafer or for each lot. In EP-A-0 443 575, by holding down the nonuniformity in film thickness by reducing the film thickness of the underlying oxide film at bottom of the thin film resistor and having a film thickness in which it is possible to obtain the largest absorption rate of the laser energy to the thin film resistor, the influence caused by the surface passivation film on top of the thin film resistor is made small, thus enabling performance of a trimming with a certain energy even if the film thickness of the surface passivation film varies due to the nonuniformity in a production process.
In recent years, there is an increasing demand for a semiconductor device which has a higher speed and is more highly integrated. Now, semiconductor elements are formed into silicon-on-insulator (SOI) layer which is isolated by insulating films such as silicon oxide. This makes it possible for the capacity between the substrate and the element to become smaller, thus realizing a higher speed, since they are isolated from each other by an insulating film Furthermore, an isolation by means of an insulating layer reduces the plane size of an element, and a lamination of elements realizes a three dimensional IC, thereby enabling higher integration.
FIG. 2 shows an example in which a thin film resistor is integrated on the semiconductor device which has said SOI structure. As shown in FIG. 2, in the semiconductor device of the SOI structure in which are formed consecutively substrate 61, embedded silicon oxide film 62, silicon layer 63 as an SOI layer, underlying oxide film 5, thin film resistor 4, aluminum electrode 7, silicon oxide film 3 and silicon nitride film 2 as a surface passivation film, when the resistance value is adjusted by laser trimming thin film resistor 4 with YAG lasers (1064 nm in wavelength), the laser beams which have passed through thin film resistor 4 are to be reflected on each interface of the surface of underlying oxide film 5, the surface of silicon layer 63, the surface of embedded silicon oxide film 62 and the surface of substrate 61. The interfering light of these reflected lights and the light incident on thin film resistor 4 each contribute to the laser trimming. However, in the case of the SOI structure as shown in FIG. 2, the light reaching thin film resistor 4 after being reflected at the interface of each layer below the resistor becomes a complicated interfering light of the reflected light at each layer. Consequently, it is very difficult to optimize the film thickness of each layer so as to control the reflected light. In particular, silicon layer 63 which composes the SOI layer is thick in its film, besides having a large refractive index but small periodic film thickness for laser light of 152 nm, and is dependent upon the SOI device which is formed therein irrespective of the laser trimming. On this account, it is difficult to control a variation in the laser energy absorption rate in a thin film resistor by means only of the film thickness.